Thermionic vacuum tube



Oct. 22, 1935. H. M. FREEMAN Er AL 2,018,257

THERMIONIC VACUUM TUBE Original Filed Jan. 8, 1923 2 Smets-Sheet l WITNESSES Oct. 242, 1935. H. M. FREEMAN Er AL THERMIONI C VCUUM TUBE 2Sheets-Sheet 2 Original Filed Jan. 8,'1923 INVENTORS eeman,eceased MFIFIR@ Hubert By Max ges Administrator E n WaZZaBce G. Wade.

Patented Cet. 2.2, 1935 UNITED STATES THERMIONIC VACUUM TUBE Hubert M.Freeman,

deceased, late of East Pittsburgh, Pa., by Max W. Reges, administrator,

Bloomfield, N. J.. delphia, Pa.:

and Wallace G. Wade, Philasaid Freeman and Wade assignors toWestinghouse Electric and Manufacturing Company, a corporation ofPennsylvania Original application January 8, 1923, Serial No.

611,263, now Patent No. Divided and this appli Serial No. 628,696

This application is a division of application Serial No. 611,263, ledJanuary 8, 1923, Patent No. 1,909,051, May 16, 1933.

Our invention relates to space-current devices and more especially tothe cathode structure of such devices.

The principal object of our invention is to provide a device of thecharacter described which may be employed for detecting, amplifying orrectifying alternating currents and which embodies a cathode structureadapted for excitation from a source of low-voltage,commercial-frequency a1ternating--currents without the introduction ofthe alternating-current noises heretofore observed in the operation ofsuch devices.

Another object of our invention is to provide a vacuum-tube structurehaving highly desirable operating characteristics, wherein a highvoltage amplification factor may be obtained while simultaneouslysecuring a comparatively low plate impedance.

A further object of our invention is to provide a vacuum-tube device ofthe class described embodying a construction which shall be adapted forquantity production methods of manufacture and which shall embody partscapable of manufacture in existing automatic machinery with minimumexpenditures of time and of money.

Heretofore, it has not been practical to employ alternating currents forthe excitation of the cathode or lament of a receiving or amplifyingtube for the reason that such currents introduce variations in the platecurrent of the tube. Such variations are thought to be due to thefollowing causes.

l. The variations in the intensity of the magnetic field established bythe alternating currents traversing the filament, thereby resulting in avariable deflection of the electron stream emanating from the filament;

2. The variations in the electric tleldaround the filament which arecaused by the reversals in the potential-distribution along thefilament;

3. The variations in the emissivity which are caused by the alternateheating and cooling of the iilament.

We have found that the desirable results outlined hereinabove may beobtained by applying a cathode construction having an operating cathodesurface which has no fall of potential along its surface, that is, aso-called equipotential surface". Such cathode surface may be renderedthermionically active in a number of different ways, as by subjectingthe same to heat or to an electron bombardment. In one form of embodi-1,909,051, May 16, 1933.

cation August 13, 1932,

13 Claims. (Cl. Z50-27) ment of our invention, we provide a cathodeconstruction comprising a central heater element and a cooperatingequipotential cathode surface which is positioned immediately adjacentto the heater element. The thermal energy of the heater element may betransferred to the cathode surface either by conduction or by radiation.

With these and other objects and applications in view, our inventionfurther consists in the combinations and details of circuit arrangementshereinafter more fully set forth and claimed and illustrated in theaccompanying drawings, wherein;

Figure l is a front elevational view of an evacuated electric deviceembodying our invention in a preferred form, a portion of the containingwalls of the envelope being broken away and the grid and plate elementsbeing shown in longitudinal section,

Fig. 2 is a side elevational view of the construction of Fig. 1.

Fig. 3 is an enlarged detailed longitudinal sectional view of thecathode construction of Figs. 1 and 2.

Fig. 4 is a view, similar to Fig. 3, showing a modification in the formof the member constituting the equipotential cathode surface and alsoshowing a modification in the means for' insulatingly supporting andseparating the branch portions of the heater element.

Fig. 5 is a view, similar to Fig. 4, showing an alternative constructionfor the equipotential cathode surface,

construction and in the 10 Fig. 6 is a view similar to Fig. 5, butshowing the member forming the operating cathode "1 surface insulatinglycarried by the heater element,

Figs. 7 and 8 are enlarged front and side detail elevational views of afurthermodied form of cathode construction embodying our invention.

Fig. 9 is a side elevational detail view illustrating our inventionapplied to a vacuum-tube construction employing a pair of controllingelements,

Figs. 10 and 11 are enlarged side elevational and top plan views of acathode construction wherein the equipotential cathode surface isenergized by thermal radiation rather than by thermal conduction, as inthe preceding ilgures.-

Fig. 12 is a diagrammatic view of circuits and apparatus embodying oneform of our invention and illustrating a circuit arrangement wherein theequipotential cathode element is connected to the heater element withinthe inclosing envelope,

Fig. 13 is a view similar to Fig. 12 but showing a circuit arrangementwherein an additional lead for the equipotential cathode element isbrought out from the tube permitting the usual vacuumtube circuits to beelectrically independent of the supply circuits for the heater element,and

v Fig. 14 is a diagrammatic view of circuits and apparatus employing thevacuum-tube construction shown in Fig. 9.

In a preferred form of embodiment of our invention, as shown in Figs, 1,2 and 3, we provide an elongated envelope I having a re-entrant portion2 terminating in a supporting press 3. A cathode construction 4comprises a mass of rei'ractory material 8 in the form of a slendersolid cylinder provided with a pair of adjacently positionedperforations 8 and 1 which extend between the ends of the cylinder 5.The perforations 8 and 1 are of such dimensions as to receive afilamentary heating element 8 which is threaded up through oneperforation and down through the other, providing a filament of invertedU-shape having parallelly extending portions 8 and Il and a top portionI2.

The distance between the parallel portions 8 and I l of the filament 8is so small that the magnetic field established by currents traversingone portion or section substantially neutralizes the magnetic fieldestablished by currents traversing the other section, thereby avoidingone of the causes for variations in the plate current of vacuum tubeswhen `employing alternating currents for the energizaltion of thecathode element. 'I'he filament 8 may be energized by connecting thesame to filament supply conductors I3 and I4 which are supported in thepress 3 and are provided with external extensions I8 and I8,respectively.

The cylindrical member 5l is preferably made of some insulatingrefractory material which, when heated to the temperature of the heaterelement 8, is free from chemical action therewith.

In the course of much experimental work, we have found that zirconpossesses such desirable characteristics.

A member I1. which forms the equipotential cathode surface, is shown inthe form of a tube having its inner surface closely embracing thetubular insulating member 8. The outer surface of the member I1 may becoated with oxides of barium, strontium or other substance which isrendered thermionically active when subjected to heat.

The cathode construction 4 may be supported at its upper end by means ofa carrier rod I8 extending from the press 3, as hereafter described. Inpractice, it has been found desirable to provide a spring connectionbetween the cathode construction 4 and the supporting carrier rod I8 inorder to provide for the expansion and contraction oi' the heatingelement 8. 'I'he desired result is obtained by welding a strip I8 to theupper end of the cathode construction, preferably to the tubular memberI1, and connecting the strip I8 and the carrier rod I 8 by means of aspring member 2|. The equipotential cathode member I1 may beelectrically connected externally of the tube I by connecting the sameto one of the supply conductors, for example, conductor I3, by means ofa conductor 22.

The cathode construction 4 may be surrounded by plate and grid elements23 and 24 of conventional design. lAs a matter of illustration, the

- grid 24 is shown in the form of a helical member with its axiscoinciding with the longitudinal axis of the heater element 8. The grid24 is supported in position by means of a carrier -rod 25 extending fromthe press 3 and welded to the several helices thereof to provideadditional stiffness to the grid construction.

The plate 23 is a tubular member symmetrically 4 positioned with respectto the previously men- 5 tioned elements and supported by carrier rods28 and 21 which are mounted in the press 3; One of the carrier rods, say21, may be extended `through the press 3 to form an external circuitterminal 28. l0

The envelope I may be provided with a base 28, which comprises a solidcircular insulating member 38 and a collar 3i, opposite ends of thelatter rigidly embracing the re-entrant tube portion 2 of the evacuatedelectric device and the 15 insulating member 38, in any approved manner.'Ihe outer end of the insulating block 30 may be provided with a raisedportion or base 32 and a plurality of hollow terminal pins 33, 34, 35and 38. The several external connections are dis- 20 posed in the hollowportions of the terminal pins such that pins 33 and 34 are connected tothe terminals of the heater element and pins 35 and 38 are connected tothe terminals of the plate and grid elements 23`and 24, respectively.The pin 25 element 33 also serves as a terminal connection for theequipotential cathode member I1.

In Fig. 4 is shown an enlarged detail longitudinal sectional view of acathode construction 31 which differs from that previously described in30 the following important respects. The adjacent sections 3 and II ofthe heater element 8 are insulating supported and spaced by means ofseparate tubular members 38 and 39, respectively. The tubular members 38and 38 may be made of 35 any refractory material having propertiessimilar to those described for the tubular supporting member 8 shown inFig. 3. The cathode construction herein shown has an equipotentialelectron-emitting element in the form of a cylindrical 4o casing 4Ihaving a cap portion 42 which is electrically connected to the bentportion I2 of the heater filament 3. 'Ihe structure may be supported bymeans of a carrier rod 43 which is welded to the cap portion 42. 'I'hecarrier rod 43 45v may be extended through the press 3 to serve also asan external terminal connection for the equipotential cathode member 4I.

In Fig. 5 is shown a cathode construction which is differentiated fromthat of Fig. 4 in the form of 50 the member constituting theequipotential cathode surface. In this construction, the cathode surfaceisy formed by spirally winding an oxidecoated platinum strip 44 aroundthe outer portions of the tubular spacing members 38 and 38. 55

The construction 45 shown in Fig. 6 is distinguishable over thepreceding cathode constructions in that the oxide-coated platinum strip44 forming the equipotential vcathode surface is wound directly aroundthe sections 8 and I I of the 60 I heater element 8, the oxide coatingon the platinum strip 44 serving to insulate the strip 44 from theheating element 8. If additional insulation is necessary, the heatingelement 8 itself may be covered by some insulating oxide, such, for ex-65 ample, as magnesium oxide. 'I'he structure may be supported by meansof a carrier rod 48 which is welded to the bent portion I2 of the heaterelement and to the strip 44. The carrier rod 48 may also serve as a leadfor the equipotential cathode 70 member 44.

In Figs. 'I and 8, a cathode construction is shown which differs fromthe foregoing constructions in the following respects. A heater element41. which may be in the lform of a fiat ribbon of 75 tungsten or othersuitable material, is doubled to form apair of adjacently positionedsections 48 and 49, as in the preceding figures. 'I'he adjacent sections48 and 49 are separated by a thin mica strip I. Equipotential cathodemembers 52 and 53, in the form of oxide-coated strips ofv nickel, arepositioned immediately adjacent to the filament sections 48 and 49,respectively, and are insulatingly spaced therefrom by mica strips 54and `65. 'I'he structure is bound tightly together at the opposite endsthereof by collar members 56 and 51 and may be supported from the top bymea-ns of a carrier rod 58 having bail extensions 59 and 6I secured tothe collar 56. The carrier rod 58 may be extended through the press toserve as an external terminal connection for the cathode members 52 and53.

Fig. 9 is a greatly enlarged view of a vacuumtube construction 62 whichis differentiated over that of Figs. 1 and 2 in the provision of anadditional controlling element 63, all as described more fullyhereinafter.

The foregoing cathode constructions are all characterized by the factthat the equipotential cathode surface thereof is energized by means ofthermal conduction, the insulating means separating the heater elementand the equipotential cathode member serving as the heat conductingmeans between the two members.

Figs. 10 and 11 are greatly enlarged views of a construction S4 whereinan equipotential cathode member 65 is heated by thermal radiation ratherthan by thermal conduction. The construction there shown comprises aheater element 66 in the form of adjacently positioned parallellyextending lamentary sections 61 and 68, the sections being adjacentlypositioned to reduce the distorting effects of the varying magneticfields established by the filament exciting currents as previouslydescribed. Adjacent ends of the filament sections are welded to asupporting member 69 of nickel or other suitable conducting material.The equipotential cathode member 65 is shown in the form of acylindrical casing inclosing the parallelly extending sections of theheater element 66 and is rigidly Asecured to the supporting block 59.The construction may be supported by means of a carrier rod 1I havingone end rigidly secured to the supporting block G9.

Fig. 12 is a diagrammatic view of the vacuumtube construction shown inFigs. 1, 2 and 3 together with circuit connections whereby the tube mayfunction as an amplifier of alternating currents. The grid 24 and theequipotential cathode I1 are connected by conductors 12 and 13,respectively, to a source of incoming signals (not shown). Aplate-filament circuit 14, which extends frorn the plate 23 to theequipotential cathode element I1, includes a detecting device and asource 1E of direct-current energy. A source of alternating-currentenergy (not shown) is operatively connected to the heater element 8through a transformer 11. It is noted that, in this arrangement,theequipotential cathode member I1 is connected to the heater element 8within the evacuated portions of the tube I, thereby requiring only fourterminal pins in the base 29.

The system shown in Fig. 13 is distinguishable over that of Fig. 12 inthe provision of separate connections for the equipotential member I1and for the heater element 8, thereby making the energizing circuits forthe heater element 8 electrically independent of the usual vacuum-tubegrid, filament and plate circuits. Such arrangement, however,necessitates the provision of an additional plug element in the base 29of the tube as 'indicated at 80 in Fig. 13.

The system shown in Fig. 14 illustrates one application of thevacuum-tube construction 62 shown in Fig. 9 'I'he additional gridelement 63 '-is connected to the equipotential cathode member I1 bymeans of a conductor 18 including a source 19 of direct-current energytendingto make the grid 63 positive with respect to the equipotentialsurface. When such condition obtains, the grid 63 serves as anelectrostatic screen tending to further decrease the tendency of thealternating exciting currents to vary the tube characteristics. When thegrid 63 is made positive, as shown in the drawings, it forms, in effect,anartificial cathode, all as will be readily understood by those skilledin the art.

In practice, we have obtained remarkably high voltage amplification withvacuum tubes employing an indirectly heated cathode surface, asdescribed in the foregoing portions of the specication. Thelow-resistance type of vacuum tube heretofore employed has a plateimpedance of from 15,000 to 25,000 ohms, with amplification factorsranging from 5 to 7, whereas the operating characteristics of avacuum-tube device embodying our invention are such that a tube may bedesigned having a plate impedance of 10,000 ohms and a voltageamplification factor of 10. Thus, it is seen that the figure of merit,

which is the ratio of the amplification factor squared to the plateresistance, is, in a vacuumtube construction embodying our invention,approximately four times greater than that of the ordinary tubesheretofore employed.

While we have shown a number of embodiments of our invention, for thepurpose of describing the same and illustrating their principles ofoperation, it is apparent that various changes and modifications may bemade in the nature and the mode of operation and in the details ofconstruction without departing from the spirit of our invention. Wedesire, therefore, that only such limitations shall be imposed thereonas are indicated by the appended claims or demanded by the prior art.

We claim as our invention:

1. A thermionic tube having therein an anode and a cathode structure andhaving external anode and cathode-terminals and terminal provisions fora cathode-energizing circuit, said cathode-structure comprisingresistance wire connected between said terminal provisions, each portionof such resistance wire being adjacent to but out of contact withanother portion of such resistance wire, which exhibits at every instantan opposite electric polarity from the first-named portion, wheneversaid terminal provisions are connected in circuit with a source ofalternating electric current, whereby external field effect of thecathode-heating current is neutralized.

2. A thermionic tube having therein a plate, a grid, and a cathode, saidcathode comprising terminals and a filamentary structure connectedbetween said terminals and comprising adjacent non-contiguous portionswhich exhibit opposite electric polarity when said terminals areconnected in circuit with the ordinary alternating current lightingsystem.

3. A cathode structure comprising a plurality of parallel non-contiguousfilaments each having terminals and supporting members connected theretoadapted to conduct energy to the said filaments, adjacent terminals ofthe said laments being connected to supporting members o! oppositepolarity.

4. A cathode assembly comprising a cylindrical casing, electron emittingmaterial on said casing, a conductive member across one end of saidcylindrical casing and connected thereto and a plurality of filamentsattached to said conductive member.

5. A cathode assembly comprising a cylindrical casing, electron emittingmaterial on said casing, a conductive member across one end of saidcylindrical casing, heating means connected to said conductive memberand a combined supporting means and cathode lead connected to saidconductive member.

6. An indirectly heated equipotential cathode assembly comprising ahollow cylindrical casing, electron emitting material on the outersurface of said casing. a conductive member across one end of saidcylindrical casing, a pair of iilaments supported on said conductivemember and extending parallel through said cylindrical casing and acombined supporting means and cathode lead connected to said conductivemember.

7. Any electrical discharge device comprising a uni-potential cathodehaving an alternatingcurrent heater, an anode, a control electrode, andan electrostatic screening means positioned between the surface of saidcathode and said anode. f

connecting said cathode and said anode, and an electrostatic screeningmeans interposed between the surface of said cathode and said anode.

l0. An electrical discharge device comprising a cathode having a surfaceof conducting material with a thermionically emissive coating at leastpartially enclosing an electrical heater for rendering said surfacethermionically emissive, an electrode external to said cathode surfaceand spaced therefrom, and an electrostatic screen between said cathodesurface and said electrode.

11. An electrical discharge device comprising a cathode having a surfaceof conducting material with a thermionicaliy emissive coating at leastpartially enclosing an electrical heater for l5 rendering said surfacethermionically emissive, an electrode external to said cathode surfaceand spaced therefrom, a control electrode and an electrostatic screenbetween said cathode and the mst-mentioned electrode.

12. An electrical discharge device comprising a cathode heated byalternating current, an anode,

a source of substantially constant voltage supply connected between saidcathode and said anode, and electrostatic screening means positionedbetween the surface of said cathode and said anode and connected to saidcathode through a current path which does not'include said source.

13. An electrical discharge device comprising a cathode heated byalternating current, an anode, a control electrode, leads for connectingsaid cathode and said control electrode to a source of varying voltage,a path for output currents responsive to said varying voltage connectedbetween said cathode and said anode, and 35 electrostatic screeningmeans positioned between the surface of said cathode and said anode andconnected to said cathode through a current path which does not includesaid rst mentioned path.

MAX F. REGES. Administrator of lthe Estate of Hubert M. Freeman,Deceased.

WALLACE G. WADE.

